Effects of Compaction on Physical Properties of Propagation Substrates

Substrates for production of seedling plugs and cuttings include loose-filled trays and stabilized paper-wrapped plugs such as Ellepots^TM.  Substrate handling, tray-filling equipment and irrigation practices can lead to variable compaction levels in substrates.  Compaction in turn affects substrate pore size, pore continuity, gas diffusion, and crop germination and rooting. The objective of this research was to develop a protocol to measure compaction in loose-filled and stabilized propagation substrates, and quantify the effects on porosity.  Tested substrates included one plug seedling mix (fine particles, with 65% peat/20% perlite/15% vermiculite by volume) and one cutting mix (coarse particles, with 70% peat and 30% perlite).  Compaction was created by using different forces to hand-fill a 128-cell plug tray or varying vacuum settlings to manufacture 50 mm Ellepot plugs.  Growing substrates were brought to container capacity using sub-irrigation.  Substrate compaction was then measured using a digital force gauge attached to a 25 mm diameter disc, by pushing the disc into the loose-filled cell to 6 mm depth or compressing the stabilized substrate by 10 mm.  In a loose-filled tray with both fine and coarse substrates, the greatest changes in physical properties occurred as compaction increased from 125 (fine substrate) or 168 (coarse) to 200 g/cm², whereby settling (loss of volume after subirrigation) decreased from approximately 1 cm to zero, water holding capacity increased from approximately 15 to 20 ml/cell, dry bulk density increased from approximately 2 to 3 g/cell, and air porosity decreased from 20% to 5% in the fine substrate and 9 to 2% in the coarse substrate.  Increasing compaction above 200 g/cm² resulted in increasing dry bulk density, and had minor impacts on water and air holding capacity.  With stabilized substrates, increased compaction from 111 to 412 g/cm² resulted in a decline in air porosity, increased water holding capacity, and increased dry bulk density.  Subsequent trials demonstrated that commercial stabilized plugs vary in compaction and porosity, and that over-filling or under-filling of loose-filled trays can result in variable porosity and plant growth.  The research protocol could be adapted as a quality control procedure for young plant production.